A polyvinyl acetal resin is widely used for many purposes, for example, an interlayer film for a laminated glass, a wash primer of metal treatment, various coating materials, an adhesive, a resin processing agent, a ceramics binder and the like, and in recent years, the range of its applications has been extended to electronic materials. Among others, a polyvinyl butyral resin is particularly suitably used for an interlayer film for a laminated glass and the like because it is superior in a film formation property, transparency, absorbency of impact energy, and adhesion to glass.
The polyvinyl acetal resin is generally produced by dehydrating and condensing a polyvinyl alcohol resin and an aldehyde compound in the presence of an acid catalyst such as hydrochloric acid as shown in Patent Document 1. And, in Patent Document 2, a method of producing a polyvinyl butyral resin in which a polyvinyl alcohol resin and a butyl aldehyde compound are mixed with a given mixing power in the presence of an acid catalyst in an aqueous solution is disclosed.
However, in the conventional method of producing a polyvinyl acetal resin, it is essential to use an acid catalyst. When the polyvinyl acetal resin produced by the conventional production method of a polyvinyl acetal resin is used for the applications of electronic materials in which the presence of impurities may cause serious problems, a process step of neutralizing an acid catalyst is needed and further an extremely troublesome process step of cleaning remaining halogen ions in the resin which have been originated from the acid catalyst, alkaline ions used in neutralization and the like to remove impurities in the resin is needed.
On the other hand, even if it is tried to react polyvinyl alcohol with aldehyde without using an acid catalyst such as hydrochloric acid, only a polyvinyl acetal resin having an acetalization degree of the order, at most, of 40 mol % can be obtained. And, even if it is tried to heat a system to elevated temperatures to accelerate an acetalization reaction, degradation of a resin such as the break of a primary chain occurs and some resins obtained are colored.
And so, a method of producing a polyvinyl acetal resin having an acetalization degree of 70 mol % or more without using an acid catalyst, which can be put to practical use, has been requested.
An esterified polyvinyl alcohol resin also has a wide array of uses. However, there were the same problems as in the method of producing a polyvinyl acetal resin since it is essential to use an acid catalyst in order to esterify a polyvinyl alcohol resin at high efficiency using the conventional method of producing an esterified polyvinyl alcohol resin.
Patent Document 1: Japanese Kokai Publication Hei-6-1853
Patent Document 2: Japanese Kokai Publication Hei-11-349629